The following disclosure relates to electrical circuits and signal processing.
Aligning the phases of two or more signals in a communication system can be useful. The signals can be information signals at multiple points in a signal path. For example, in a communications system that uses feedback, the phase of a feedback signal may need to be aligned with the phase of a forward path signal for the system to operate correctly or efficiently. Aligning two signals is equivalent to making the relative rotation between the signals substantially zero.
A relative rotation (for example, between two complex signals) can have several causes. In a conventional application, a first complex signal can be used to modulate a radio-frequency (RF) carrier. The modulated carrier, an RF signal, undergoes analog processing, after which the modulated carrier can be demodulated. A second complex signal results from the demodulation. Any difference in phase between the modulated carrier before the processing and the modulated carrier after the processing is manifested as a relative rotation between baseband constellations corresponding to the first and second complex signals.
An example of a component in a communications system that uses feedback is a Cartesian feedback transmitter. In a conventional Cartesian feedback transmitter, a complex feedback signal is subtracted from a complex input signal to produce a complex error signal. The complex error signal is amplified and filtered to produce an intermediate signal, which is then modulated for transmission. The modulated signal is also demodulated in the transmitter to produce the complex feedback signal. Using Cartesian feedback in a transmitter improves the linearity of the transmitter, but properly aligning the phases of the complex intermediate signal and the complex feedback signal is important for stable operation.
The complex feedback signal typically has a different phase than the complex intermediate signal because of, for example, delays in the RF signal path or a phase difference between the oscillator signal used during modulation and the oscillator signal used during demodulation. A change in output power level or a change in carrier frequency can also cause a relative rotation between the complex intermediate signal and the complex feedback signal. The phase of the complex intermediate signal can be adjusted (e.g., by using a rotator circuit) to align the complex intermediate signal and the complex feedback signal. The adjustment of the phase of the complex intermediate signal can be controlled based on, for example, an estimate of the relative rotation between the complex intermediate signal and the complex feedback signal.
One technique that can be used to estimate the phase difference between the complex intermediate signal and the complex feedback signal is to multiply the in-phase component of the complex intermediate signal (Ifwd) by the quadrature component of the complex feedback signal (Qfb) and to multiply the quadrature component of the complex intermediate signal (Qfwd) by the in-phase component of the complex feedback signal (Ifb), all multiplication being done in the analog domain. The second product (Qfwd Ifb) is then subtracted from the first product (Ifwd Qfb), and the result is integrated. A rotator circuit can use the integrated result to rotate the phase of the complex intermediate signal with respect to the complex feedback signal.